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United States Patent |
5,245,104
|
Cullick
|
September 14, 1993
|
Method and apparatus for producing and separating diamondoid compounds
from natural gas streams
Abstract
A method and apparatus are disclosed for recovering and separating
diamondoid compounds from a produced natural gas stream containing
dissolved diamondoids.
Inventors:
|
Cullick; Alvin S. (Dallas, TX)
|
Assignee:
|
Mobil Oil Corporation (Fairfax, VA)
|
Appl. No.:
|
628958 |
Filed:
|
December 17, 1990 |
Current U.S. Class: |
585/812; 585/800; 585/803 |
Intern'l Class: |
C07C 007/00; C07C 007/14 |
Field of Search: |
585/812,803
|
References Cited
U.S. Patent Documents
3082211 | Apr., 1957 | Green | 585/815.
|
4666456 | May., 1987 | Thijssen et al. | 23/296.
|
4952747 | Aug., 1990 | Alexander et al. | 585/803.
|
4952748 | Aug., 1990 | Alexander et al. | 585/803.
|
4952749 | Aug., 1990 | Alexander et al. | 585/803.
|
Primary Examiner: Pal; Asok
Assistant Examiner: Achutamurthy; P.
Attorney, Agent or Firm: McKillop; Alexander J., Santini; Dennis P., Furr, Jr.; Robert B.
Claims
What is claimed is:
1. A method for recovering and separating diamondoid compounds from a
natural gas stream having diamondoid compounds dissolved therein
comprising the steps of:
(a) providing a natural gas reservoir containing a recoverable
concentration of diamondoid compounds dissolved therein;
(b) withdrawing a natural gas stream containing diamondoid compounds from
said natural gas reservoir at production wellhead pressure;
(c) depressuring and heating said natural gas stream to maintain said
diamondoid compounds in solution with said natural gas;
(d) transferring said depressured natural gas stream of step (c) to a first
precipitation zone while controlling temperature and pressure to maintain
said diamondoid compounds in solution with said natural gas upstream of
said first precipitation zone;
(e) cooling said depressured natural gas stream of step (d) within said
first precipitation zone to a temperature sufficient to evolve a
triamantane-enriched solid precipitate;
(f) collecting said triamantane-enriched solid precipitate within said
first precipitation zone in the substantial absence of diamantane-enriched
precipitate;
(g) discontinuing natural gas flow to said first precipitation zone;
(h) flushing said first precipitation zone with a solvent to dissolve said
triamantane-enriched solid precipitate, said solvent being characterized
by boiling point range at atmospheric pressure of from about 70 to about
125.degree. F.;
(i) collecting said triamantane-enriched solvent of step (h);
(j) recovering triamantane-enriched solid from said collected solvent of
step (i);
(k) transferring said cooled depressured natural gas stream of step (e)
containing essentially no triamantane to a second precipitation zone while
controlling temperature and pressure to retain the remaining diamondoid
compounds in solution with said natural gas upstream of said second
precipitation zone;
(l) cooling said depressured natural gas stream of step (d) within said
second precipitation zone to a temperature sufficient to evolve a
diamantane-enriched solid precipitate;
(m) collecting said diamantane-enriched solid precipitate within said
second precipitation zone in the substantial absence of
triamantane-enriched precipitate;
(n) discontinuing natural gas flow to said second precipitation zone;
(o) flushing said second precipitation zone with a solvent to dissolve said
diamantane-enriched solid precipitate, said solvent being characterized by
boiling point range at atmospheric pressure of from about 70 to about
125.degree. F.;
(p) collecting said diamantane-enriched solvent of step (o); and
(q) recovering diamantane-enriched product from said collected solvent of
step (p).
2. The process of claim 1 further comprising accumulating said
triamantane-enriched precipitate on a porous solid within said first
precipitation zone.
3. The process of claim 1 further comprising accumulating said
diamantane-enriched precipitate on a porous solid within said second
precipitation zone.
4. The process of claim 1 further comprising providing metallic or
nonmetallic solid inert particles within said first and said second
precipitation zones to facilitate precipitation.
5. The process of claim 1 wherein said solvent is selected from the group
consisting of carbon disulfide, chloroform, and one or more hydrocarbons
having a boiling endpoint of less than about 125.degree. F. at atmospheric
pressure.
6. The process of claim 5 wherein said solvent is carbon disulfide.
7. The process of claim 1 wherein said inert gas is carbon dioxide.
8. The process of claim 1 wherein said recovery step (j) further comprises
evaporating said solvent to produce triamantane-enriched solids.
9. The process of claim 1 wherein said recovery step (q) further comprises
evaporating said solvent to produce diamantane-enriched solids.
Description
BACKGROUND OF THE INVENTION
This invention relates to the controlled recovery of certain components
from hydrocarbon streams. It more particularly refers to recovering and
separating diamondoid organic compounds from hydrocarbon streams
containing such, and specifically to precipitating these diamondoid
compounds in a manner which facilitates their separate recovery.
Many hydrocarbonaceous mineral streams contain some small proportion of
diamondoid compounds. These high boiling, saturated, three-dimensional
polycyclic organics are illustrated by adamantane, diamantane, triamantane
and various side chain substituted homologues, particularly the methyl
derivatives. These compounds have high melting points and high vapor
pressures for their molecular weights and have recently been found to
cause problems during production and refining of hydrocarbonaceous
minerals, particularly natural gas, by condensing out and solidifying,
thereby clogging pipes and other pieces of equipment. For a survey of the
chemistry of diamondoid compounds, see Fort, Jr., Raymond C., The
Chemistry of Diamond Molecules, Marcel Dekker, 1976.
In recent times, new sources of hydrocarbon minerals have been brought into
production which, for some unknown reason, have substantially larger
concentrations of diamondoid compounds. Whereas in the past, the amount of
diamondoid compounds has been too small to cause operational problems such
as production cooler plugging, now these compounds represent both a larger
problem and a larger opportunity. The presence of diamondoid compounds in
natural gas has been found to cause plugging in the process equipment
requiring costly maintenance downtime to remove. On the other hand, these
very compounds which can deleteriously affect the profitability of natural
gas production are themselves valuable products.
The problem of diamondoid deposition and plugging in natural gas production
equipment has been successfully addressed by a controlled solvent
injection process. U.S. Pat. No. 4,952,748 to Alexander and Knight teaches
the process for extracting diamondoid compounds from a hydrocarbon gas
stream by contacting the diamondoid-laden hydrocarbon gas with a suitable
solvent to preferentially dissolve the diamondoid compounds into the
solvent. Allowed U.S. patent application Ser. No. 489,111, filed Mar. 6,
1990, to Cullick and Roach teaches a method for locating the solvent
injection point within the natural gas wellbore.
Further studies have revealed that separation of the diamondoid compounds
from the diamondoid-enriched solvent is complicated by the fact that
numerous diamondoid compounds boil in a narrow range of temperatures
surrounding the boiling range of the most preferred solvents. U.S. Pat.
Nos. 4,952,747 and 4,952,749 to Alexander et al. as well as allowed U.S.
application Ser. No. 358,761, filed May 26, 1989 teach various methods of
concentrating diamondoid compounds in the solvent for, among other
reasons, recycling the lean solvent fraction for reuse Each of these
processes produces an enriched solvent stream containing a mixture of
diamondoid compounds. While the produced diamondoid mixture may prove
useful itself, it is presently desirable to segregate the various
diamondoid homologs for further processing. Thus it would be highly
desirable to provide a method for preventing diamondoid deposition and
plugging in hydrocarbon gas production and processing equipment which
effectively separates the produced diamondoid mixture into fractions
enriched in a single diamondoid homolog.
SUMMARY OF THE INVENTION
The present invention provides a method for recovering diamondoid compounds
from a natural gas stream and for separating these compounds into
fractions which are each enriched in a single diamondoid homolog.
Thus the invention provides a method for recovering diamondoid compounds
from a hydrocarbon gas stream in a plurality of diamondoid fractions,
wherein each recovered diamondoid fraction is enriched in a selected
diamondoid homolog.
The method of the invention comprises the steps of:
(a) providing a natural gas reservoir containing a recoverable
concentration of diamondoid compounds dissolved therein;
(b) withdrawing a natural gas stream containing diamondoid compounds from
said natural gas reservoir at production wellhead pressure;
(c) depressuring and heating said natural gas stream to maintain said
diamondoid compounds in solution with said natural gas;
(d) transferring said depressured natural gas stream of step (c) to a first
precipitation zone while controlling temperature and pressure to maintain
said diamondoid compounds in solution with said natural gas upstream of
said first precipitation zone;
(e) cooling said depressured natural gas stream of step (d) within said
first precipitation zone to a temperature sufficient to evolve a
triamantane-enriched precipitate;
(f) collecting said triamantane-enriched precipitate within said first
precipitation zone in the substantial absence of diamantane-enriched
precipitate;
(g) discontinuing natural gas flow to said first precipitation zone;
(h) flushing said first precipitation zone with a solvent to dissolve said
triamantane-enriched precipitate, said solvent being characterized by
boiling point range at atmospheric pressure of from about 70.degree. to
about 125.degree. F.;
(i) collecting said triamantane-enriched solvent of step (h);
(j) recovering triamantane-enriched product from said collected solvent of
step (i);
(k) transferring said cooled depressured natural gas stream of step (e) to
a second precipitation zone while controlling temperature and pressure to
retain the remaining diamondoid compounds in solution with said natural
gas upstream of said second precipitation zone;
(l) cooling said depressured natural gas stream of step (d) within said
second precipitation zone to a temperature sufficient to evolve a
diamantane-enriched precipitate;
(m) collecting said diamantane-enriched precipitate within said second
precipitation zone in the substantial absence of triamantane-enriched
precipitate;
(n) discontinuing natural gas flow to said second precipitation zone;
(o) flushing said second precipitation zone with a solvent to dissolve said
diamantane-enriched precipitate, said solvent being characterized by
boiling point range at atmospheric pressure of from about 70.degree. to
about 125.degree. F.;
(p) collecting said diamantane-enriched solvent of step (o); and
(q) recovering diamantane-enriched product from said collected solvent of
step (p).
The process may further comprise accumulating said diamondoid-enriched
precipitate on porous solids within said precipitation zones of steps (d)
and (l). The porous solid may comprise any suitable inert porous material
which remains thermally stable under the broad ranges of temperature and
pressure specified for the precipitation zone. One examples of a suitable
porous solid is a metallic filter, i.e., a sintered metal filter.
Solvents useful in the present invention must readily dissolve diamondoid
compounds such as diamantane and triamantane. These solvents must also
remain liquid under typical ambient conditions but readily vaporize under
atmospheric pressure at temperatures slightly above ambient. Examples of
suitable solvents include naphtha, carbon disulfide, and chloroform, with
carbon disulfide being the most preferred solvent. Of the useful naphtha
boiling range solvents, naphthenic hydrocarbon mixtures enriched in
cyclohexane are preferred, and neat cyclohexane is particularly preferred.
If the selected naphtha comprises a mixture of components, the naphtha
should preferably have an endpoint of less than about 125.degree. F.
The invention further provides an apparatus for recovering a plurality of
diamondoid-enriched solid fractions from a gas stream having a mixture of
diamondoid compounds dissolved therein, said apparatus comprising:
(a) a first precipitation stage containing at least two precipitation zones
piped and valved together in parallel with a common inlet and a common
outlet for selectively flowing a gas stream through one of said
precipitation zones in a parallel/swing operational mode or through both
of said precipitation zones simultaneously in parallel;
(b) a second precipitation stage containing at least two precipitation
zones piped and valved together in parallel with a common inlet and a
common outlet for selectively flowing a gas stream through one of said
precipitation zones in a parallel/swing operational mode or through both
of said precipitation zones simultaneously in parallel;
(c) valved conduit operatively connecting said first precipitation stage
outlet with said second precipitation stage inlet;
(d) a first precipitation stage temperature controller for maintaining
temperature of said first precipitation stage within a first preselected
range;
(e) a second precipitation stage temperature controller for maintaining
temperature of said second precipitation stage within a second preselected
range;
(f) an inert gas source for selectively flowing inert gas through at least
one precipitation zone of said first and said second precipitation stages;
(g) at least one solvent reservoir with associated solvent supply valved
conduit for selectively passing solvent through at least one precipitation
zone of said first and said second precipitation stages to dissolve
diamondoid-containing solids deposited therein whereby said solvent is
enriched in diamondoid compounds; and
(h) a first solvent evaporator for collecting and evaporating enriched
solvent which has been circulated through said first precipitation stage;
(i) a second solvent evaporator for collecting and evaporating enriched
solvent which has been circulated through said second precipitation stage;
and
(j) conduit for returning condensed solvent from said first solvent
evaporator and said second solvent evaporator to said solvent reservoir.
DESCRIPTION OF THE DRAWING
The FIGURE is a simplified schematic flow diagram illustrating the major
processing steps of the present invention.
DETAILED DESCRIPTION
The present invention provides a method and apparatus for recovering
diamondoid compounds from a hydrocarbon gas stream, e.g., a produced
natural gas stream. The process of the invention may optionally be
operated in a diamondoid production mode or in a diamondoid
production/separation mode. If the diamondoid production mode is selected,
a single precipitation stage is employed under temperature conditions to
precipitate substantially all of the diamondoid compounds dissolved in the
natural gas stream. This mode is useful for producing solid mixtures of
diamondoid compounds which reflect the diamondoid composition of the
produced natural gas stream. However, in certain circumstances, it is
desirable to segregate the diamondoid compounds by structure, i.e., to
separate higher adamantane homologs from lower adamantane homologs, and
specifically to separate diamantane from triamantane. The invention
accomplishes this object by segregating triamantane in a first
precipitation stage from diamantane in a second precipitation stage by
tailoring conditions within the precipitation stages to selectively
precipitate solids enriched in these diamondoid materials. As used herein,
the term "adamantane homolog" designates the substituted and unsubstituted
polycyclic alkanes having a skeletal structure resembling diamond,
examples of which include adamantane, diamantane, and triamantane, as well
as their substituted and functionalized derivatives.
Process Flow
The production/separation mode of the present invention is schematically
shown in the FIGURE, and parallel/swing operation is described below. It
is to be understood, however, that if necessary, both parallel circuits
can be operated simultaneously.
Referring now to the FIGURE, a diamondoid-containing natural gas stream is
withdrawn from a producing natural gas well 10 through line 11 at wellhead
pressure which generally ranges from about 2,000 to about 15,000 psig,
most typically around about 6,000 psig, and temperature of from about
200.degree. F. to about 300.degree. F. The produced natural gas stream is
depressured across choke valve 12 to a pressure of from about 900 to about
1400 psig, typically around 1200 psig. To avoid uncontrolled precipitation
of solids, particularly diamondoids, due to Joule-Thompson cooling across
the choke, the natural gas lines and valves downstream from the choke are
heat traced, e.g., with steam or electric heat tracing (not shown). The
heat tracing suitably maintains the natural gas lines downstream from the
choke at temperatures in the range of about 200.degree. F. to about
260.degree. F. The depressured natural gas stream then flows through choke
outlet line 13 and is teed into lines 14 and 16, which are suitably valved
15, 17 to charge the diamondoid-containing natural gas stream to at least
one of two temperature-controlled precipitations zones, G1 and G2, which
are configured for parallel/swing operation as described in greater detail
below.
To collect a triamantane-enriched solid in the first precipitation zone,
G1, valve 15 is set open, allowing depressured diamondoid-containing
natural gas to enter first precipitation zone G1 which is in contact with
cooler Al. Precipitation zone G1 comprises a tubular conduit which
preferably contains a porous solid material to facilitate triamantane
precipitation. Examples of suitable porous solids include metallic
filters, for example a sintered metal filter. Examples of other suitable
porous solids include, metal balls or metal filings. Inert porous
nonmetallic solids may also be used, for example, alumina balls.
Temperature control in first precipitation zone G1 is critical for
effective diamondoid separation in the present invention. For this reason,
it is preferred that the total heat capacity of cooler A1 be relatively
large compared to the required cooling load in first precipitation zone
G1. To precipitate triamantane-enriched solids in the first precipitation
zone G1, the temperature is preferably controlled at around 190.degree. F.
The cooler A1 and precipitation zone G1 can be of any suitable
configuration and it is preferred that the precipitation zone G1 and
cooler A1 be constructed as a shell-and-tube heat exchanger. In this
configuration, the precipitation zone G1 most preferably comprises a
coiled tube immersed in a circulating water bath contained within the
shell of the heat exchanger.
The natural gas stream is then withdrawn from first precipitation zone G1
through line 20 which is equipped with valve 22. As precipitate
accumulates within first precipitation zone G1, the available paths for
gas flow are constricted and pressure drop across the precipitation zone
gradually increases. Thus pressure drop across the precipitation zone is a
useful indicator of precipitate loading. When the pressure drop reaches a
predetermined level, typically about 1-2 psi, but not exceeding 10 psi,
natural gas flow through the first precipitation zone G1 is interrupted
and the natural gas is routed through the second precipitation zone G2
which is piped in parallel with first precipitation zone G1. Valve 15 is
closed, and valve 17 is opened, shifting flow from line 14 to line 16.
Temperature conditions within the second precipitation zone G2 are
equivalent to those in the first precipitation zone G1 during the
precipitation step.
The accumulated triamantane-rich precipitate is then collected from the
first precipitation zone G1. First, the zone is purged with a cold inert
gas such as vaporized carbon dioxide to cool the precipitation zone and to
remove natural gas from the precipitation zone and associated lines. This
purging step is accomplished by withdrawing liquid carbon dioxide
contained in C1 and stored at about 800 psi. The liquid is withdrawn into
lines 30 and 34 by opening valve 32. With G1 now open to the flare (at
atmospheric pressure), the liquid carbon dioxide is flashed across valve
35, which is a needle valve or other valve whose effective orifice size
can be controlled to ensure that line 34 remains liquid filled until after
the cooling step. By Joule-Thompson cooling, the vaporized carbon dioxide
is at a temperature well below 40.degree. F. The cool carbon dioxide gas
stream enters and flushes natural gas from first precipitation zone G1
which is then exhausted to flare via line 38 through open valve 39. The
flushing continues until the temperature of G1 reaches about 70.degree. F.
During the inert gas flushing step, the circulating water in cooler A1 is
preferably drained to a level below that of the precipitation zone G1 to
facilitate cooling of the precipitation zone G1 by the cool vaporized
inert gas.
Following the inert gas purge, the first precipitation zone G1 is flushed
with a solvent such as carbon disulfide, chloroform, or a naphtha having
an endpoint of less than about 125.degree. F, as described above. Solvent
flows from fresh solvent reservoir F1 through line 40 to solvent pump D1.
The solvent pump must generate sufficient pressure to overcome the
pressure drop in the precipitate-filled precipitation zone G1 and to
return the enriched solvent to enriched solvent evaporator E1.
The lean solvent flows from solvent pump D1 through line 42 and open valve
44 into line 30. Valve 32 is closed and valve 35 is open, allowing solvent
to flow through line 34 and subsequently through line 14 into the first
precipitation zone G1. Valves 39 and 22 are closed and the solvent,
enriched in triamantane, returns to solvent evaporator E1 through line 46
and open valve 47. Solvent flow through the first precipitation zone G1
continues until substantially all of the triamantane-enriched precipitate
is dissolved and removed. This generally requires between about 1 and
about 3 precipitation zone volumes of fresh solvent.
When the precipitation zone rinse step is completed, the solvent evaporator
E1 temperature is elevated to a temperature sufficient to boil off the
solvent while retaining solid triamantane-enriched material within the
solvent evaporator. Typical solvent evaporator temperatures for the
solvents referred to above are about 120.degree. F. to about 175.degree.
F. If carbon disulfide is used as the solvent, the solvent evaporator is
heated to a temperature of from about 125.degree. to about 175.degree. F.,
typically about 120.degree. F., near the carbon disulfide boiling point
115.3.degree. F., to evaporate the solvent, leaving behind the
triamantane-enriched solids. The evaporated solvent returns to fresh
solvent reservoir F1 via line 60 and open valve 61 for storage and reuse.
Line 60 is connected to flare (not shown) via line 64 which is equipped
with valve 65. Valve 65 is typically a pressure relief valve. Line 60 may
optionally include a cooler or condenser if required to liquify the
solvent for storage and reuse. During the precipitation zone rinse step,
partially enriched solvent may optionally be recirculated through the
precipitation zone G1 to dissolve the accumulated precipitate. At the
conclusion of the precipitation zone rinse step, solvent is flushed out of
the precipitation zone with the inert gas so that when natural gas flow to
the precipitation zone is restarted, the precipitation zone is filled with
inert gas.
When the precipitation zone G1 has been flushed with solvent and the
enriched solvent is collected in solvent evaporator E1, the diamondoid
solids are recovered by evaporating the solvent as described above. The
solvent evaporator vessel is then opened and triamantane-enriched solids
are removed. The solvent evaporator vessel may suitably comprise any
commercially available heated vessel configuration, but preferably
comprises a drum or kettle surrounded by an external heating coil.
External heating coils are preferred to facilitate physical removal of the
accumulated diamondoid solids.
Natural gas flow continues through the second precipitation zone G2 and is
withdrawn through line 50 and open valve 52 until a pressure drop across
the zone is noted as described above with reference to the first
precipitation zone G1. Flow is then shifted back to the first
precipitation zone G1 while the second precipitation zone is purged with
inert gas and triamantane-rich precipitate is recovered via solvent
circulation as described above with reference to the first precipitation
zone G1. Specifically, triamantane-enriched solids deposited in the second
precipitation zone G2 dissolve in the circulating solvent which is
withdrawn from fresh solvent reservoir F1 and charged through solvent pump
D1 through line 42 and open valve 44 as described above. Valve 35 is
closed, and the solvent flows through line 36 and open valve 37, through
line 16 into the second precipitation zone G2. The triamantane-enriched
solvent is withdrawn from the second precipitation zone G2 through line
50. Valve 52 is closed, diverting flow from line 50 through line 58 and
open valve 59 to line 46 which returns the triamantane-enriched solvent to
the solvent evaporator E1.
The natural gas stream, now containing diamantane but essentially no
triamantane, is withdrawn from the first precipitation stage and is
charged to at least one of the diamantane precipitation zones of the
second precipitation stage.
The operation of the second precipitation stage is similar in principle to
that of the first with the exception that the third and fourth
precipitation zones, G3 and G4, are controlled at a lower temperature than
the first two precipitation zones, G1 and G2. Specifically, during
operation, the temperature of precipitation zones G3 and G4 is controlled
within the range of about 100.degree. to about 110.degree. F.
The natural gas stream flows from the first precipitation stage, where
triamantane-enriched precipitate was removed, to the second precipitation
stage, where the stream is cooled to recover diamantane-enriched
precipitate. Lines 20 and 50 are teed into line 70 which conveys the
natural gas stream from the first precipitation stage to the second
precipitation stage. Line 70 tees into lines 72 and 74, which are equipped
with valves 73 and 75, respectively. The second precipitation stage
comprises two parallel precipitation zones, G3 and G4, operates in a
continuous/swing mode similar to that of the first precipitation stage.
Thus to route natural gas through the third precipitation zone G3, valve
73 is open and the natural gas stream flows through line 72 into the third
precipitation zone G3.
Temperature within the third precipitation zone G3 ranges from about
100.degree. to about 110.degree. F., and is typically about 100.degree.
F., controlled by cooler A3. Diamantane-enriched solids precipitate in the
third precipitation zone G3 and the purified natural gas stream is
withdrawn via line 80 and open valve 82. When pressure drop across the
third precipitation zone G3 has increased to about 1-2 psi, natural gas
flow is discontinued by closing valve 73, and the third precipitation zone
G3 is cooled and purged with a cold vaporized inert gas, preferably
vaporized carbon dioxide as described above with reference to the first
precipitation zone G1. For the inert gas purge, valve 92 is open and valve
104 is closed, allowing the inert gas to vaporize from a second inert gas
reservoir C2, flowing through lines 90 and 94, through the third
precipitation zone G3. The mixture of inert gas and natural gas withdrawn
from the third precipitation zone G3 through line 80 is sent to flare (not
shown) via line 84 and open valve 85.
Solvent circulation in the second precipitation stage is substantially
identical to that of the first precipitation stage as described above.
Solvent from fresh solvent reservoir F2 flows through line 100 to solvent
circulation pump D2, through line 102 and open valve 104 and into line 90.
To route the solvent through the third precipitation zone G3, valve 97 is
closed and the solvent flows through line 94 and open valve 95 to line 72
which charges the solvent to the inlet of the third precipitation zone G3.
The diamantane-enriched solids deposited in the third precipitation zone
G3 readily dissolve in the circulating solvent which is returned to a
second evaporator E2 through line 86 and open valve 87.
During the inert gas purge and solvent circulation steps in the third
precipitation zone G3, natural gas flows through the fourth precipitation
zone G4 for continuous recovery of diamantane-enriched solids. Natural gas
flow through the fourth precipitation zone G4 continues until the pressure
drop across the fourth precipitation zone G4 reaches a predetermined
level, at which time natural gas flow is shifted back to the third
precipitation zone G3. In a manner similar to the procedures described
above, the fourth precipitation zone G4 is then purged with cold vaporized
inert gas from the second inert gas reservoir C2, which flows through
lines 90 and 96, through the open valve 97 and into line 74 where it is
charged to the inlet of the fourth precipitation zone G4. The mixture of
natural gas and inert gas is then withdrawn from the fourth precipitation
Zone G4 and flared via line 114 and open valve 116. Next, solvent is
circulated through the fourth precipitation zone G4 from fresh solvent
reservoir F2 through solvent circulation pump D2, through line 102 and
open valve 104 and into line 90. To route the solvent through the fourth
precipitation zone G4, valve 95 is closed and the solvent flows through
line 96 and open valve 97 to line 74 which charges the solvent to the
inlet of the fourth precipitation zone G4. The triamantane-enriched solids
deposited in the fourth precipitation zone G4 readily dissolve in the
circulating solvent which is returned to a second evaporator E2 through
line 120. The evaporated solvent then returns to fresh solvent reservoir
F2 as described above with reference to fresh solvent reservoir F1. The
diamantane-enriched solids are then recovered from solvent evaporator E2
as described above with reference to triamantane-enriched solids and
solvent evaporator E1. The purified natural gas stream is then withdrawn
from the second stage of the diamondoid recovery apparatus via line 130
for further processing and sale.
EXAMPLE
Natural gas as characterized in Tables 1 and 2, below, is produced from a
wellhead at a rate of 15 MMSCFD (million standard cubic feet per day) at
6000 psia and 260.degree. F. The natural gas stream is charged through a
choke to reduce pressure to about 1200 psia while heat tracing on the
choke and the associated lines maintains the average natural gas
temperature downstream of the choke at 260.degree. F. The natural gas
stream is charged to a diamondoid precipitation unit comprising two sets
of two controlled temperature precipitation zones piped in parallel for
continuous/swing operation for two-stage diamondoid precipitation. The
four zones are piped in a series/parallel configuration such that one zone
of each parallel pair is available for diamondoid precipitation and one
zone is available for solvent flushing for dissolution of the accumulated
diamondoids.
TABLE 1
______________________________________
Example Diamondoid Concentration in Dry Natural Gas Sample
Concentration
Concentration
Component lbs/mmscf ppm
______________________________________
adamantane 15 300
1-methyladamantane
30 600
1,3-dimethyladamantane
18 360
1,3,5-trimethyladamantane
4 80
diamantane 9 180
4-methyldiamantane
5 100
1-methyldiamantane
6 120
triamantane 2 40
methyltriamantane
1 18
______________________________________
TABLE 2
______________________________________
Example Dry Natural Gas Composition (after processing)
Mole Percent
______________________________________
hydrogen sulfide
7.1
nitrogen 0.4
methane 88.0
ethane 0.8
carbon dioxide 3.6
propane 0.1
______________________________________
The first two precipitation zones are maintained at a temperature of
190.degree. F. and pressure of 1200 psia to selectively precipitate from
the natural gas stream a mixture of solids highly enriched in triamantane.
The first two precipitation zones are contained in 316L stainless steel
tubular conduits having length of 4 feet and an inside diameter of 4
inches. Each stainless steel conduit contains a sintered stainless steel
filter (dimensions: 4 inch nominal inside diameter and 2 foot length) for
collecting triamantane-enriched precipitate. Thus the total volumetric
capacity of the first precipitation zone is about 13,000 cc, due to the
fact that the porosity of the filter is less than about 1. The maximum
capacity of the first precipitation zone is about 34 pounds of
triamantane, but pressure drop across the first precipitation zone
requires that the zone be flushed with solvent after about 9 pounds of
triamantane have accumulated. For the stated natural gas compositions and
flowrates, about 2.6 pounds of triamantane and methyltriamantane
precipitate per million standard cubic feet of natural gas flow upon
cooling from 260.degree. to 190.degree. F. at the operating pressure of
1200 psig. Small amounts of other diamondoids also precipitate, but the
resulting solid is highly enriched in triamantane, thus demonstrating the
effectiveness of separating diamondoid fractions by the present method.
The stainless steel conduit is immersed in a constant temperature water
bath at 190.degree. F. When the pressure drop across the first
precipitation zone reaches 1-2 psi, (generally after accumulating about 9
pounds of triamantane, or after about 1.5 hours of natural gas flow at 15
MMSCFD), the natural gas flow through the first precipitation zone is
discontinued and the first precipitation zone is purged with cold
vaporized carbon dioxide until the temperature of the first precipitation
zone reaches 70.degree. F. The mixture of carbon dioxide and purged
natural gas is vented to flare. Next, carbon disulfide (b.p. 46.3.degree.
C., 115.3.degree. F.) is circulated through the first precipitation zone.
The accumulated triamantane typically requires about 4 pounds of solvent
per pound of triamantane dissolved, or about 6 gallons of solvent for
every 9 pounds of triamantane. The precipitation zone is therefore flushed
three times with a total of about 10 gallons of solvent.
The enriched solvent is then displaced to a heated evaporator vessel at
temperature of 120.degree. F. and the resulting vapors are condensed and
recovered for reuse, leaving triamantane-enriched solids in the evaporator
vessel. The evaporator vessel is then periodically opened and
triamantane-enriched solids are physically removed.
When the solvent rinse is completed, the first precipitation zone remains
out of service until the second precipitation zone requires solvent rinse.
The second precipitation zone, identical in configuration to the first, is
then charged with natural gas and the triamantane recovery stage is
continued.
The natural gas effluent from the triamantane recovery stage is then
charged to a second, lower temperature diamantane precipitation stage. The
second stage comprises two precipitation zones (the third and fourth
precipitation zones) configured for parallel/swing operation as described
above with reference to the triamantane recovery stage. In the diamantane
recovery stage, however, more precipitation zone volume is required. Thus
the third and fourth precipitation zones are contained in 316L stainless
steel tubes as described above with the exception that the tubes for the
third and fourth precipitation zones have inside diameters of
approximately 8 inches, with each of the third and fourth precipitation
zones thus providing maximum diamantane precipitation capacity of about
136 pounds.
Upon entering the diamantane recovery stage, the natural gas stream is
routed to either the third or the fourth precipitation zone where it is
cooled to an average temperature of 100.degree. F. at a pressure of 1200
psia. Diamantane solids contained in the natural gas stream precipitate
and form a solid within the third precipitation zone, depositing about 8.6
pounds of diamantane-enriched solids per million standard cubic feet of
natural gas processed.
The stainless steel conduit is immersed in a constant temperature water
bath at 100.degree. F. When the pressure drop across the first
precipitation zone reaches 1-2 psi, (generally after accumulating about 34
pounds of diamantane, or after about 1.5 hours of natural gas flow at 15
MMSCFD), the natural gas flow through the third precipitation zone is
discontinued and the third precipitation zone is purged with cold
vaporized carbon dioxide until the temperature of the third precipitation
zone reaches 70.degree. F. The mixture of carbon dioxide and purged
natural gas is vented to flare. Next, carbon disulfide (b.p. 46.3.degree.
C., 115.3.degree. F.) is circulated through the third precipitation zone.
The accumulated diamantane typically requires about 4 pounds of solvent
per pound of diamantane dissolved, or about 6 gallons of solvent for every
9 pounds of diamantane. The precipitation zone is therefore flushed three
times with a total of about 10 gallons of solvent.
The enriched solvent is then stored in a heated evaporator vessel at
temperature of 120.degree. F. and the resulting vapors are condensed and
recovered for reuse, leaving diamantane-enriched solids in the evaporator
vessel, which is periodically opened for diamantane removal as described
above.
When the solvent rinse of the third precipitation zone is completed, the
third precipitation zone remains out of service until the fourth
precipitation zone requires solvent rinse. The fourth precipitation zone,
identical in configuration to the third, is then charged with natural gas
and the diamantane recovery stage is continued.
Changes and modifications in the specifically described embodiments can be
carried out without departing from the scope of the invention which is
intended to be limited only by the scope of the appended claims.
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